Seamless steel pipes have been manufactured by means of a Mannesmann plug-mill process, a Mannesmann-mandrel mill process, a Mannesmann-Assel mill process, a Mannesmann-push bench mill process, and the like. These processes comprise piercing a solid-core billet heated to a predetermined temperature in a heating furnace by a piercing mill to form a hollow bar-like hollow piece, reducing mainly the wall thickness thereof by an elongator such as a plug mill, a mandrel mill, an Assel mill or a push bench mill, in order to form a hollow shell, and reducing mainly the outer diameter thereof by a reducing mill such as a sizer or a stretch reducer to form a seamless steel pipe of a predetermined dimension.
The present invention-relates to an elongation rolling process that is the second step of such a seamless pipe manufacturing process. Although the present invention will be described hereinafter based on the Mannesmann-mandrel mill process, the same effect can also be obtained in an elongation rolling process of the other pipe manufacturing processes.
FIG. 1 is a view showing the Mannesmann-mandrel mill process, wherein (a) shows a rotary hearth heating furnace, (b) a piercer (piercing mill), (c) a mandrel mill (elongator), (d) a reheating furnace and (e) a stretch reducer (reducing mill).
In the past, a full-floating-type mandrel mill was generally used to continuously roll a hollow shell 2 with a mandrel bar 1 through grooved rolls 3, wherein the mandrel bar 1 was inserted inside of the hollow shell, as shown in (c) of FIG. 1, Recently, a semi-floating-type mandrel mill (also called a restrained mandrel mill) becomes common as a mandrel mill of further high efficiency and high quality.
FIG. 2 is a comparative view of the full-floating-type mandrel mill and the semi-floating-type mandrel mill, wherein (a) shows the full-float mandrel mill and (b) the semi-floating-type mandrel mill.
The semi-floating-type mandrel mill shown in (b) of FIG. 2 includes a full-retract system where the mandrel bar 1 is hold and constrained by a mandrel bar retainer 4 up to the end of rolling, and pulling back the mandrel bar 1 simultaneously at the end of rolling, and a semi-float system for releasing the mandrel bar 1 simultaneously at the end of rolling. Generally, the full-retract system is adopted for manufacturing middle size seamless pipes, and the semi-float system for manufacturing small size seamless pipes.
In the full-retract system, an extractor is connected to the outlet side of the mandrel mill, and a hollow shell is pulled out during rolling by the mandrel mill. If the temperature of the pipe at the outlet side of the mandrel mill is sufficiently high, the pipe is pulled out by a sizing mill or stretch reducer instead of the extractor, which results in be reduced to a final target dimension without reheating.
A lubricant is applied onto the surface of the mandrel bar for the purpose of reducing the friction between the pipe's inner surface and the mandrel bar surface to prevent scratching of the pipe's inner surface and sticking flaws on the mandrel bar surface, and also for the purpose of easy stripping of the mandrel bar after elongation rolling.
In the past, water-soluble oil based on heavy oil containing fine powdery graphite was used as the lubricant, or fine powdery graphite was sprayed onto the surface of an oil-coated mandrel bar and used as the lubricant.
Recently, a non-graphitic lubricant called borax, a scale-melting agent, is increasingly used as a smokeless lubricant. At the time of elongation rolling of the stainless steel pipes and high alloy steel pipes, particularly, mica-based non-graphitic lubricants can be used.
The following Patent Document 1 discloses a method for manufacturing a small size seamless pipe, characterized by diameter-reducing and elongating a hollow shell made by piercing in a cold rolling process. In this method, a hot elongation rolling process by the use of the mandrel mill is omitted. However, this omission is only for simplifying the pipe manufacturing process, not for preventing the carburization of pipes in the hot elongation rolling process by use of mandrel mill. There is no description about the prevention of carburization in the Patent Document 1.
[Patent Document 1]                Japan Patent Unexamined Publication No. H10-58013        
When a stainless steel pipe or high alloy steel pipe is elongated by use of the mandrel mill, a carburization phenomenon occurs on the inner or outer surfaces of a resulting pipe product, particularly, on the inner surface thereof. Carburization has an undesirable influence such as deterioration of corrosion resistance on the pipe product. This carburization phenomenon is extremely troublesome, which occurs in the use of non-graphitic lubricants or graphitic lubricants. The previous use of graphitic lubricants results in graphite fine powder being suspended in the air of a pipe factory and adhering to the mandrel bar.